1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle, and in particular, to a technique that is effective for preventing generation of a counter electromotive voltage in a motor during cruising.
2. Description of the Related Art
Conventionally, hybrid vehicles which carry motors as power sources for driving vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In parallel hybrid vehicles, the motor connected to the engine assists the rotation of the drive shaft of the engine while charging a battery using a generator, which is the motor itself.
In parallel hybrid vehicles, a variety of control operations are executed so that the motor assists the engine at the time of acceleration and a battery is charged by deceleration regeneration at the time of deceleration, in order to maintain the residual capacity of the battery constant and to respond to the driver""s demands.
Japanese Unexamined Patent Application, First Publication No. Hei 7-123509, for example, discloses a hybrid vehicle which controls power generation by the motor, when the sum total of electric power obtainable by regeneration, which is estimated from the present driving state, and the present remaining charge of the power storage unit is smaller than a predetermined standard charge.
This hybrid vehicle has an advantage in that the loss of regenerative power or degradation of the fuel consumption due to excess power generation can be prevented by optimizing the target charge of the power storage unit based on parameters such as the vehicle speed correlated with the kinetic energy.
However, as the vehicle speed increases, the kinetic energy of the vehicle increases and the electric power obtained by regeneration increases, so that the battery charge at the time of cruising must have the capacity to absorb the excess charge.
However, the motor used for the hybrid vehicles or the like generally generates a counter electromotive voltage in response to the number of rotations. Thus, if this counter electromotive voltage is higher than the voltage of the power storage unit, a current flows from the motor to the power storage unit. Therefore, it becomes necessary to perform switching control by a power drive unit in order to protect the power storage unit from the excess voltage.
On the other hand, the switching control of the counter electromotive voltage causes the generated power by the motor to be dissipated as heat and a load is applied to the engine due to power generation friction.
In particular, the above problem becomes remarkable when a capacitor is used, because it has a comparatively small energy capacity and its voltage decreases as the energy decreases, in contrast to a battery, which has a large energy capacity and a stable voltage.
The present invention was made to solve the above problems and the object of the present invention is to efficiently prevent generation of the counter electromotive voltage by the motor at the time of cruising.
The first aspect of the present invention provides a control system for a hybrid vehicle with an engine and a motor, which are power sources, and a power storage unit for supplying electric power to the motor and for storing the electric power by regeneration, comprising: a rotational number detecting device for detecting the rotational number associated with said motor, a target voltage setting device for setting the charge target voltage of said power storage unit in response to the output of said rotational number detecting device, and a charge control device for controlling the charge of said power storage unit, wherein said charge control device controls the charge of said power storage unit such that the charge of said power storage unit reaches said charge target voltage when the voltage of said power storage unit is below said charge target voltage.
By constituting the control system of the hybrid vehicle as shown above, when the present voltage of the power storage unit less than target charge voltage, the control system carries out an immediate charging operation of the power storage unit. Thus, the relationship of the present voltage of the power storage unit less than the counter motor electromotive voltage can be maintained most of the time. Accordingly, it is possible to obtain a high fuel efficiency.
According to the second aspect of the present invention, in the above control system of a hybrid vehicle, said motor is disposed on the same driving shaft connecting said engine with the driving wheels for producing an assist driving force to assist the output from said engine or for producing energy by regeneration in response to the driving state of said vehicle and the charge state of said power storage unit, and, when the vehicle is in a charge mode, in which, when said vehicle does not require the assistance of the driving power by said motor, the control system operates said motor as a power generator for charging said power storage unit, said charge control device controls the charge of said power storage unit such that the charge of said power storage unit reaches said charge target voltage.
By constituting the control system as described above, because the voltage of the power storage unit is maintained above the voltage of the counter electromotive voltage, and the unnecessary loads on the engine due to adjusting the counter electromotive voltage can be eliminated, the degradation of the fuel consumption by application of the load to the engine can be avoided.
According to the third aspect of the present invention, in the above control system of a hybrid vehicle, said power storage unit is constituted by a capacitor comprising an electric double layer capacitor.
By constituting the control system of a hybrid vehicle as described above, the power storage unit of the present invention does not have to use device such as a battery, which is dependent on the chemical reactions and the response characteristics of the present invention are improved.